A pressure reducing valve with manually adjustable setting using a screw and spring, is known. However, with this known pressure reducing valve, once the control pressure is set, it remains constant during operation. In some applications, the control pressure needs to be varied according to the requirement of the application. See the co-pending, commonly assigned U.S. patent application Ser. No. 09/222,948, for CONTINUOUSLY VARIALBE TRANSMISSION WITH CONTROL ARRANGEMENT AND METHOD FOR REDUCING TRANSMISSION BELT SLIPPAGE filed concurrently with this application and hereby incorporated by reference, for a detailed disclosure of such an application.
The variable pulley transmission, or continuously variable transmission (CVT) as it is commonly called, has been under development for two decades, its use as a power transmission is mainly for automotive applications. Its control is rather complicated and is very sensitive to operating conditions. For example, in a control arrangement for a variable transmission disclosed in U.S. Pat. No. 4,458,318, a variable line pressure is controlled and applied to the sheave of the secondary or driven pulley of the transmission, and also serves as the hydraulic power supply to a ratio control valve in a speed control loop of the transmission. Due to the variable supply pressure in this known continuously variable transmission, both the pressure control valve (a pressure relief type valve) and the ratio control valve gains change with the supply pressure, and the control is therefore very sensitive to system operating conditions.
There is a need for an improved continuously variable transmission with a control arrangement including a pressure reducing valve for reduction of belt slippage in the transmission which overcomes these drawbacks and disadvantages of known pressure control valves and continuously variable transmissions. More particularly, there is a need for an improved pressure reducing valve and continuously variable transmission with control arrangement using the same which are simpler and more robust than those disclosed in U.S. patent Ser. No. 4,458,318.
It is an object of the present invention to provide a pressure reducing valve and a continuously variable transmission with control arrangement using the valve which meet this need.
Another object of the present invention is to provide a pressure reducing valve and a continuously variable transmission with control arrangement using the valve for use as a constant speed drive for an aircraft electrical power generator where belt slippage can be minimized or eliminated.
These and other objects are attained by the pressure reducing valve of the invention which, is a two-stage pressure reducing valve comprising a pilot stage control valve providing an output control pressure and flow of a fluid to a main stage valve from a hydraulic pressure source passing through the pilot stage control valve in response to an input signal applied to the pilot stage control valve from an external source. The control valve includes a fixed orifice through which the flow of fluid from the source passes, and a variable orifice downstream of the fixed orifice in a flow passage through the pilot stage control valve. The opening of the variable orifice is controlled by the input signal applied to the pilot stage control valve from the external source for varying the output control pressure provided by the pilot stage control valve.
The pressure reducing valve of one embodiment further comprises, as the main stage valve, a two-way spool valve having a valve body and a valve spool movable within the valve body in response to an imbalance of forces on the valve spool. The valve body has a supply port for receiving pressurized fluid from the source of hydraulic pressure and a load port for communicating a load pressure and flow of pressurized fluid from the spool valve to a hydraulic load. The open area of the load port is dependent on the position of the valve spool within the valve body. The valve body has a first passage communicating the control pressure and flow from the pilot stage control valve to one end of the spool valve and a second passage communicating a feedback pressure and flow from the load pressure from the spool valve to the other end of the valve spool. The position of the valve spool within the valve body depends on the pressure difference applied on the two ends of the valve spool whereby the load pressure from the spool valve to the hydraulic load can be made proportional to the input signal applied to the pilot stage control valve.
The pressure reducing valve can be operated in either an open-loop or a closed-loop configuration to provide simple, effective control of the output control pressure and flow to the hydraulic load in response to the input signals applied to the pilot stage control valve.
In another form of the invention, the main stage spool valve is a three-way spool valve with the valve body comprising a drain port for discharging pressurized fluid from the spool valve to lower the load pressure from the spool valve. In this embodiment, the open area of the supply port and the open area of the drain port are controlled by the position of the valve spool within the valve body.
The pilot stage control valve in a first embodiment of the invention is a linear solenoid valve which is controlled by a voltage applied to a solenoid thereof as the input signal for varying control pressure. In another form of the invention, the pilot stage control valve is a torque motor controlled single-jet flapper valve having a torque motor and a flapper valve whose position is controlled by the motor for varying the distance the flapper valve is away from the variable orifice thereby controlling the opening thereof in response to a voltage applied to the torque motor as the input signal applied to the pilot control control valve.
A continuously variable transmission according to the invention for transferring drive from an engine to a device to be driven, comprises a primary pulley for receiving drive from an engine, a belt, a secondary pulley which is coupled over the belt of the primary pulley for transferring drive to a device to be driven. The primary and secondary pulleys each have a fixed sheave and an axially movable sheave with a hydraulically operated actuator therfor to effect ratio change of the transmission and to maintain belt tension. A single source of constant hydraulic pressure is operatively connected for driving each of the actuators. The single source of constant hydraulical pressure is connected to the actuator of the primary pulley by way of a ratio control valve and is operatively connected to the actuator of the secondary pulley by way of a pressure reducing valve according to the invention. In the preferred embodiment, the transmission drives an electric generator of an aircraft where the pressure reducing valve is part of a hydraulic pressure control loop for controlling the hydraulic pressure applied to the actuator of the secondary pulley by way of the pressure reducing valve as a function of the load from the generator on the transmission and the pulley pitch radius of the secondary pulley.
These and other objects, features and advantages of the present invention will become more apparent from the following detailed description of several embodiments of the invention taken with the accompanying drawings.